1. Field of the Invention
The present invention relates to a process for producing a prepreg in which a thermosetting resin in a liquid state is supplied as a matrix resin to a sheet-shaped reinforcing substrate which comprises a fabric, and the substrate including the resin is heated, and also to an apparatus for producing the prepreg by such a process. Particularly, the present invention relates to a process for producing a prepreg in which a sheet-shaped reinforcing fabric substrate such as a glass cloth is impregnated with a matrix resin without using any solvent, and also to an apparatus for producing the prepreg by such a process.
2. Description of the Prior Art
For the production of a prepreg, a process has been known in which a thermosetting resin as a matrix resin is dissolved into a solvent so as to have a low viscosity solution, a reinforcing substrate such as a glass cloth is impregnated with the solution, and the impregnated substrate is dried. Using such a process, since the matrix resin sufficiently penetrates into the reinforcing substrate by means of the capillarity, the prepreg can be produced relatively easily.
In the above process in which the solvent is used, there are the following problems: solvent recovery facilities are necessary, and explosion preventive facilities are necessary, which increase the installation cost for the production of the prepreg; and a trace amount of residual solvent in the prepreg results in the quality deterioration of the produced prepreg because of the formation of bubbles.
In order to overcome the problems relating to the process in which the solvent is used, Japanese Patent Kokai Publication (JP-A) No. 5-200748 discloses a process for producing a prepreg using an apparatus as shown in FIG. 27. In the process, a double belt press 21 is provided, and a feeding unit 22 for a reinforcing substrate 1 such as a glass cloth and an extruder 23 are disposed in front of an inlet of the double belt press 21. In the extruder 23, a matrix resin 2 such as an epoxy resin is melted and extruded in a film form onto the reinforcing substrate so as to coat the substrate with the resin. The laminar composite of the reinforcing substrate and the resin matrix is introduced into the double belt press 21 and heated while applied with a plane pressure (i.e. a pressure applied by a plane(s)). During this stage, the matrix resin melts and penetrates into the inside of the reinforcing substrate when the composite is passed through the double belt press, so that the prepreg is produced containing the matrix resin in a semi-hardened state.
However, even when the above process using such an apparatus as shown in FIG. 27 is employed, there are still problems as follows:
When using the double belt press, the plane pressure is applied to the composite onto the both sides thereof so as to impregnate the reinforcing substrate with the matrix resin, so that bubbles do not easily get out of the inside of the composite and the bubbles are likely to be left, whereby voids are formed in the prepreg. Particularly, when the substrate has a larger woven mesh (or "metsuke") so that impregnability of the resin into the substrate is insufficient, the voids are more likely to be left. In addition, since the plane pressure are applied onto the both sides of the composite, the matrix resin on the reinforcing substrate is likely to be spread toward both edge portions of the composite so that an adjustable range of a content of the matrix resin which the substrate includes therein and thereon is narrow and it is difficult to produce a prepreg having a larger matrix resin content Further, an amount of the matrix resin in one side portion of the prepreg may be different from that in the other side portion, and surface smoothness of the prepreg may be insufficient.